That would actually be the perfect theme for a DLC, but it would still work if
Karlach's heart got fixed, imo...
Kinda' like in Mass Effect, the more you do at the beginning, the more you get near the ending, could swing into buffs, resistances, a lotta good stuff...
As for what the motivation would be in this case, well... pure payback sounds plenty good to me... I mean
Zariel messed with pretty much the whole team by the game's ending, I'd bet Wyll, Lae'zel, maybe even Astarion and Shadowheart (after her change of heart - yet again, I apologise) would be game to stomp some Arch-Daemon behind
